This invention relates to the production of polyhydroxyalkanoate in plants.
Poly-3-hydroxybutyrate (PHB) is a linear polyester of D(-)-3-hydroxybutyrate. It was first discovered in Bacillus megaterium in 1925. Polyhydroxybutyrate accumulates in intracellular granules of a wide variety of bacteria. The granules appear to be membrane bound and can be stained with Sudan Black dye. The polymer is produced under conditions of nutrient limitation and acts as a reserve of carbon and energy. The molecular weight of the polyhydroxybutyrate varies from around 50,000 to greater than 1,000,000, depending on the micro-organisms involved, the conditions of growth, and the method employed for extraction of the polyhydroxybutyrate. Polyhydroxybutyrate is an ideal carbon reserve as it exists in the cell in a highly reduced state, (it is virtually insoluble), and exerts negligible osmotic pressure.
Polyhydroxybutyrate and related polyhydroxyalkanoates, such as poly-3-hydroxyvalerate and poly-3-hydroxyoctanoate, are biodegradable thermoplastics of considerable commercial importance.
The terms "polyhydroxyalkanoate" and "PHA" as used hereinafter include polymers of 3-hydroxybutyrate, polymers of related hydroxyalkanoates such as 3-hydroxyvalerate, 3-hydroxyhexanoate, 3-hydroxyoctanoate, 3-hydroxydecanoate, and also copolymers and mixtures of more than one of these hydroxyalkanoates.
Polyhydroxyalkanoate is biodegradable and is broken down rapidly by soil micro-organisms. It is thermoplastic (it melts at 180.degree. C.) and can readily be moulded into diverse forms using technology well-established for the other thermoplastics materials such as high-density polyethylene which melts at around the same temperature (190.degree. C.). The material is ideal for the production of biodegradable packaging which will degrade in landfill sites and sewage farms. The polymer is biocompatible, as well as biodegradable, and is well tolerated by the mammalian, including human, body; its degradation product, 3-hydroxybutyrate, is a normal mammalian metabolite. Polyhydroxybutyrate degrades only slowly in the body making it suitable for medical applications where long term degradation is required.
Polyhydroxyalkanoate, produced by the micro-organism Alcaligenes eutrophus, is manufactured, as a copolymer of polyhydroxybutyrate and polyhydroxyvalerate, by Imperial Chemical Industries PLC and sold under the Trade Mark BIOPOL. The nature of the polymer, for example the proportions of PHB and PHV is determined by the substrate supplied in the fermentation. It is normally supplied in the form of pellets for thermoprocessing. However, polyhydroxyalkanoate is more expensive to manufacture by existing methods than, say, polyethylene. It is, therefore, desirable that new, more economic production of polyhydroxyalkanoate be provided.